When a module is loaded from bytecode and then the value of
`use-compiled-file-paths' changes, an attempt to load a submodule
would fail, because source isn't used if the main module is
already declared, and the bytecode code is not used according to
`use-compiled-file-paths'. Make the bytecode path stick when it
is used once, so that submodule loads succeed, and make it work
even with `namespace-module-attach'.
The module-attach part of this protocol requires a change to the
API of a module name resolver: the notification mode gets two
arguments, instead of one, where the second argument is an
environment.
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